3rd generation partnership project (3GPP) mobile communication systems based on a wideband code division multiple access (WCDMA) radio access technology are widely spread all over the world. High-speed downlink packet access (HSDPA) that can be defined as a first evolutionary stage of WCDMA provides 3GPP with a radio access technique that is highly competitive in the mid-term future. However, since requirements and expectations of users and service providers are continuously increased and developments of competing radio access techniques are continuously in progress, new technical evolutions in 3GPP are required to secure competitiveness in the future. Reduction of cost per bit, increase of service availability, flexible use of frequency bands, simple structure and open interface, proper power consumption of a user equipment, and the like are defined as requirements.
To overcome a restriction on a service area and a limitation in a user capacity, the wireless communication system provides a communication service by dividing the service area into a plurality of cells. This is referred to as a multi-cell environment. A cell is a region in which a base station (BS) provides a communication service. One BS can provide a service to at least one cell. A user equipment (UE) belongs to one cell. A cell to which the UE belongs is referred to as a serving cell. Another cell adjacent to the serving cell is referred to as a neighbor cell.
The wireless communication system differs from a wired communication system in that a seamless service has to be transmitted to UEs having mobility. That is, when a UE moves its location from a serving cell to a neighbor cell, the neighbor cell to which the UE moves has to be switched to the serving cell in order to provide the seamless service to the UE.
As such, a process of switching the serving cell of the UE due to movement of the UE is referred to as handover. In this case, a cell to which the UE originally belongs is referred to as a source cell, and a new cell to which the UE moves is referred to as a target cell. A BS providing a communication service to the source cell is referred to as a source BS. A BS providing a communication service to the target cell is referred to as a target BS. In the handover process, the UE is disconnected from the source BS and is then newly connected to the target BS.
Since the wireless communication system is a time-variant system, a radio condition may change over time. In addition, when the UE moves, a radio condition between the serving BS and the UE and a radio condition between the target BS and the UE may rapidly deteriorate. In addition, when a relay station is employed between the BS and the UE, a channel state between the relay station and the BS may also deteriorate. Therefore, those variables have to be considered as much as possible to ensure quality of service and to effectively use radio resources when a cell is selected or when data is transmitted and received after the cell selection.
In addition, the present invention prevents waste of radio resource or transmission delay by selecting a relay station on the basis of only signal strength.
In addition, the present invention intends to increase efficiency of radio resource usage by selecting a suitable relay station (or base station) by considering its state that varies on a real-time basis.